Chronology Current Month Current Thread Current Date [Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] Starlight

Maybe I'm missing the point of the original posting, but it appears to me that the "frequency" seen to the photon is an unanswerable question. Light is created, travels to a receiver (maybe close - maybe light years away), and ceases to exist. The proper time elapsed for the photon is zero, i.e. the journey is basically instantaneous. This would seem to make counting oscillations a mute point.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Ken Caviness
Sent: Wednesday, October 27, 2010 11:18 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Starlight

More precisely: What looks like a B field to one observer looks like a
combination of E & B fields to another observer (one in motion with
respect to first), and what looks like an E field to the first observer
looks like a combination of E & B fields to the second observer.

What looks like _only_ a B field to one observer does _not_ look like
only an E field to any other observer.

This is closely related to the spacetime interval in relativity. If
one observer sees an interval as entirely space, no observer will see
it as entirely time, and vice versa.

This is not a coincidence, since Maxwell's equations are invariant
under the Lorentz transformation, the transformation of coordinates in
special relativity. They have the same form for all observers,
although each observer uses her own distance & time scales, etc.
Electromagnetic waves, oscillating E- & B-fields moving at the speed c,
are valid solutions for Maxwell's equations. All inertial observers
see electromagnetic waves as oscillating E-&B-fields, moving at the
speed of light.

At first glance it seems like a copout to say, "No-one can travel at
the speed of light, so it makes no sense to ask what light looks like
to the light itself." But it may be the best answer possible. We
_can_ say that no matter what your speed is (with respect to any
convenient reference frame), 0.9c, 0.99c, 0.999c, 0.9999c,..., you will
_still_ see all light as composed of oscillating E-&B-fields, moving at
the speed of light -- with respect to _you_. No matter how closely you
approach the speed of light (again, with respect to any convenient
reference frame), you will never see these E-&B-fields stop or even
travel with a different speed.

Of course, the frequency of the oscillation _is_ different according to
different observers (Doppler shift). So the color of the light may be
different (red/blueshifted). An observer speeding along the path of
the light, starting at the distant star and heading for earth, will
still see the light ray moving much faster than him (at speed c, of
course), and because of length contraction may see the distance
traveled as very, very small, so that the trip might only take a
fraction of a nanosecond (according to his clocks), maybe not even one
complete period of the oscillation of the light. (According to earth
observers the trip time was long, but the moving observer's clocks were
running slow because of time dilation.)

In any case, it is not fair to consider the E- & B-fields as stationary
even in the limit, they always oscillate, for all inertial observers.

All the best,

Ken Caviness
Physics

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Josh Gates
Sent: Wednesday, October 27, 2010 7:27 AM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] Starlight

http://www.xkcd.com/811/

So... how oscillations of the magnetic and electric fields does the
light "see"
during the journey? None? I know that what looks like a B field to
one looks
like an E field to another, but what about the oscillating fields of
a light wave -
what do they look like to the light?

Thanks,
Josh
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l